Spacecraft such as satellites employ a number of sensors to determine their relative orientation in space. An accurate indication of the spacecraft orientation is important for properly controlling the positioning of various spacecraft components, such as solar wings or communication antennas. Typically, spacecraft attitude measurements characterize spacecraft orientation relative to the pitch, roll, and yaw axes of the spacecraft. Depending upon the particular application, the spacecraft orientation measurements may be indicated directly by appropriate sensors or derived indirectly from those sensors.
Momentum bias spacecraft utilize a momentum wheel which, typically, spins about the pitch axis so as to create a momentum bias nominally along that axis. This momentum bias provides "gyroscopic stiffness" to resist roll and yaw axis disturbance torques which attempt to perturb the spacecraft pitch axis from orbit normal. Thus, a larger momentum bias provides greater gyroscopic stiffness but also makes controlling the spacecraft more difficult.
The gyroscopic effect also couples the roll and yaw dynamics. Prior art spacecraft control systems have recognized and utilized this coupling to control both the roll and yaw axes while directly sensing only the roll axis and deriving a yaw measurement from the roll sensor. The efficacy of this control strategy is proportional to the magnitude of the momentum bias. Thus, for a relatively small momentum bias (typically 20-80 N-m-sec on a geosynchronous communications satellite), with no direct sensing of the yaw attitude, the yaw attitude cannot be controlled very accurately.
At present, commercially available geosynchronous momentum bias satellites do not have a device for direct measurement of the satellite yaw attitude. Instead, these satellites utilize the control strategy described above to derive a yaw attitude measurement from a roll sensor. The derived yaw attitude measurement is then utilized by the control system to properly position the satellite or its components. For example, the derived yaw attitude measurement may be used to point a satellite communication antenna toward a predetermined target.
Since this control strategy requires a compromise between the size of the momentum bias and the accuracy of the spacecraft pointing, this system typically has limited accuracy and is sensitive to significantly large time-varying disturbance torques. Such a system is inadequate for use in a data relay spacecraft with large slewing antennas which experiences large time-varying disturbances due to the antenna slews, and the like. These applications require accurate yaw pointing in order to provide antenna pointing service to target locations which are off Earth nadir.
Commercially available satellites, including data relay spacecraft, often include an analog sun sensor. The analog sun sensors are typically mounted on the solar wings of the spacecraft. These sun sensors provide a coarse yaw measurement during a large portion of the day, while an Earth sensor provides roll and pitch measurements. However, due to their location on the solar wings, the analog sun sensors are subject to bias and distortions. More importantly, near solar equinox, there are periods spanning close to two hours occurring twice daily when no yaw measurement is available. This occurs because, during those periods, the line from the spacecraft to the sun is nearly along the spacecraft yaw axis.
Many satellites have been flown with digital, rather than analog, sun sensors as part of their attitude measurement systems. For example, the standard architecture for NASA's low-Earth-orbit mapping satellites includes a digital sun sensor. The digital sun sensor is typically a redundant component which functions only as a backup in case one of the two star sensors fails.